/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Create a new MeshDescription PolygonGroup and return it's ID. PolygonGroups are not the same as Polygroups, they essentially represent Mesh Sections, which then reference Material Slots, etc
[FPolygonGroupID](API\Runtime\MeshDescription\FPolygonGroupID) AppendPolygonGroup ( [FName](API\Runtime\Core\UObject\FName) MaterialSlotName )